This is what happens during a US government shutdown

(CNN) -- If both houses of Congress are unable to pass a funding proposal -- which, at this point, looks very possible -- the United States federal government will shut down Friday.

Lawmakers had a similar deadline last month, but they passed a continuing resolution December 21 to keep the federal government funded for another month.

Why The Government Might Shut Down

Now, Democrats are pushing a solution for recipients of the Deferred Action for Childhood Arrivals program and insist that it be addressed before the next deadline Friday -- and it's not clear whether the two parties will reach an agreement by then.

The House voted Thursday night to avert a government shutdown, sending the bill on to the Senate, where its future is much less certain.

Shortly before the House vote, the Freedom Caucus said a majority of its members would vote to support a stopgap spending measure, a key sign that holdout conservatives who had been undecided earlier had come on board.

Whether the Senate can pass such a measure ahead of a Friday midnight deadline is a different issue altogether. Because the measure will need 60 votes to pass the chamber to break a filibuster, Republican leaders need as many as more than a dozen Democrats.

What Happens When The Government Shuts Down?

A shutdown, however, doesn't mean every federally funded agency, program and service will grind to a halt. Whoever works for agencies and departments that are considered nonessential, including agencies that pay out small business loans and process passport requests, will cease to work until Congress is able to agree on a bill for the federal budget.

The employees in these departments would be placed on "furlough." In previous shutdowns, everyone who stayed home was paid retroactively after an agreement was reached in Washington.

At the peak of the 2013 government shutdown, about 850,000 employees were furloughed per day, according to the Office of Management and Budget.

But What Is Considered Essential And Non-essential?

If the government were to shut down, the military is considered essential and would still report for duty. However, the troops -- including those in combat -- will potentially not be paid during a shutdown. In addition, many civilian Department of Defense employees will not be working during a shutdown, including instructors at military academies and maintenance contractors.

Special counsel Robert Mueller's team would continue to operate, a Justice Department spokesperson told CNN.

"All employees with the Special Counsel's Office are considered exempt and would continue their operations in the case of a lapse in appropriations," the spokesperson said.

National Parks Will Close

But if you had plans for a vacation to visit any national parks, zoos or museums, those will be closed.

The shutdown would also affect the Bureau of Alcohol, Tobacco, Firearms and Explosives, meaning if you wanted a gun permit, you'll have to wait until the shutdown is over.

Essential services, such as Social Security, air traffic control, and the Transportation Security Administration, will continue to be funded even if some employees of those agencies are not.

And the US Postal Service won't stop serving residents -- you'll still get your mail.

Congress Still Gets Paid

Members of Congress will continue to receive paychecks because it's written into law.

In 2013, the shutdown especially affected residents of Washington. But this time around, Mayor Muriel Bowser vowed that services in the city will continue, unlike last time.

"Washington, D.C., is open," Bowser said in a statement published Friday. "DC government will continue to provide services to our residents, the services they expect and deserve uninterrupted.

She also said the city plans to help the federal government maintain the National Mall if there's a shutdown.

"I've called on my agencies, where we are able, to step in for the federal government," she said during the press conference. "The National Mall is operated by the National Park Service and there are many other National Service Park properties throughout Washington, DC ... we will step in and ensure litter and trash are picked up along the National Mall to keep nation's front yard clean of debris."

This story has been updated to reflect new information from the District of Columbia.